1. Field of the Invention
The present invention relates to a quadrature mixer circuit applied to a wireless receiver, and more particularly, to a quadrature mixer circuit applied to a zero intermediate frequency (IF) type (a so-called direct conversion type) wireless receiver or a low IF type wireless receiver.
2. Description of the Related Art
While super heterodyne type wireless receivers have excellent noise figure (NF) characteristics, the super heterodyne type wireless receivers have a large number of components including local oscillators, image removing filters and an IF band-pass filter, which is an obstacle for incorporating a radio frequency (RF) portion and a baseband portion into one chip.
In order to decrease the number of components, various direct conversion type wireless receivers have been developed. In this case, the improvement of quadrature mixer circuits applied to such direct conversion type wireless receivers is indispensable.
In a first prior art quadrature mixer circuit using two-input mixers (see: FIG. 29 of JP-A-9-205382), since a local oscillator signal has the same frequency as that of a radio frequency (RF) signal, a DC offset cannot be completely removed, which requires DC offset removing circuits. Also, trouble in reception sensitivity may be generated. Further, the reception sensitivity of other wireless receivers may be suppressed. This will be explained later in detail.
Even in a second prior art quadrature mixer circuit using two-input mixers and a local oscillator signal having a half frequency of the RF signal, the same disadvantages as the first prior art quadrature mixer circuit exist. This also will be explained later in detail.
In a third prior art quadrature mixer circuit (see: JP-A-9-205382 & Takafumi Yamaji et al, “An I/Q Active Balanced Harmonic Mixer with IM2 Cancelers and a 45° Phase Shifter”, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, pp. 2240–2246, December 1998), two-input even-ordered harmonic mixers, a voltage controlled oscillator having a frequency different from the frequency of the RF signal and a π/4 phase shifter are provided. In the third prior art quadrature mixer circuit, however, it is difficult to realize the π/4 phase shifter. This also will be explained later in detail.